While stronger regulations have resulted in cars that are far cleaner than those of three decades ago, the air in many American cities remains dangerous to breathe. The reason: Americans are driving more miles than ever before and that additional driving is spurred in part by the expansion of America's highway network, which is one of the major causes of suburban sprawl. More Highways, More Pollution finds that American cities with the largest highway networks per capita also tend to be those with the greatest air pollution and warns that further highway expansion could lead to additional air pollution and threats to public health.